1. Field of the Invention
The present invention relates to a titanium alloy and a process for producing the same. The titanium alloy has high strength and good workability and hence is suitable for such applications as aircraft engine and golf club face which need good mechanical properties including high strength, ductility, and toughness.
2. Description of the Related Art
Among high-strength titanium alloys are so-called near-xcex2 alloys typified by Ti-10V-2Fe-3Al and Ti-5Al-2Sn-2Zr-4Mo-4Cr. These titanium alloys undergo xcex2-process so that they have good balanced strength and toughness. This process consists of heating a titanium alloy above the xcex2-transus and then subjecting it to plastic deformation before the xcex1 phase precipitates, so that a large number of precipitation sites are introduced into xcex2 grains. This process prevents the xcex1 phase from preferentially precipitating at the grain boundary, which would otherwise precipitate to degrade strength after cooling or aging, and also permits the acicular microstructure to develop all over in the subsequent heat treatment. This process is designed basically to make the xcex2 phase undergo work hardening by plastic deformation, while suppressing the precipitation of the xcex1 phase during plastic deformation, and then cause the xcex1 phase to precipitate in the uncrystallized xcex2 matrix at an adequate temperature below the xcex2-transus.
On the other hand, if an ingot of titanium alloy is to be forged, it has to be heated again because it is usually cooled. Unfortunately, reheating is not allowed after the xcex2 process because it destroys the previous sub-structure. Therefore, the titanium alloy has to be formed into a shape by plastic deformation which can be finished in a short time by single heating. This poses a problem with low yields.
Among other high-strength titanium alloys are xcex2 alloys typified by Ti-15Mo-5Zr-3Al and Ti-15V-3Cr-3Sn-3Al. These titanium alloys are superior in cold-workability and are capable of precipitation hardening due to precipitation of the xcex1 phase from the metastable xcex2 phase by aging. Since these titanium alloys enable cold-rolling before aging, strips can be produced most efficiently by sequential steps of hot rolling (as in the case of commercially pure titanium strips), coiling, optional solution treatment, and cold rolling and annealing (at a temperature close to that of solution treatment), by making use of the feature of xcex2 titanium alloys.
However, xcex2 alloys such as Ti-15Mo-5Zr-3Al highly liable to age hardening experience additional age hardening due to remaining heat after hot rolling and coiling to such an extent that the coiled strips cannot be recoiled. A conceivable way to avoid this trouble is by batch-annealing in the coiled state. This is not desirable, however. From the standpoint of strength after aging treatment, it is desirable to perform cold working further while keeping the work-hardened conditions caused by hot-rolling, or preferably without annealing, so that the fine uniform xcex1 phase is precipitated. Annealing above the xcex2-transus causes recrystallization and grows grains, and annealing below the xcex2-transus causes the xcex1 phase to precipitate. Thus, annealing greatly impairs the subsequent cold workability and the strength after aging treatment.
To avoid this trouble, it is necessary to employ the so-called sheet rolling method for Ti-15Mo-5Zr-3Al which is in general use today. (Sheet rolling is intermittent operation that hinders productivity.)
Because of their high strength, near-xcex2 alloys and xcex2 alloys are used for aircraft engine parts and golf club face which need high strength. These titanium alloys, however, pose a problem when they undergo age hardening to increase strength. That is, if they are hot-rolled at a higher temperature, their xcex2 microstructure becomes so coarse as to bring about extreme embrittlement. Therefore, they have to be hot-rolled at a lower temperature. However, this is difficult to practice with the existing facilities on account of the limited rolling load. The present practical way of making sheets from high-strength near-xcex2 alloys or xcex2 alloys is the so-called sheet rolling which is capable of rolling at a low temperature without requiring recoiling as mentioned above. This process is extremely poor in productivity.
The above-mentioned problem with near-xcex2 alloys and xcex2 alloys stems from the fact that it is desirable for the alloy to have a high degree of supersaturation and to precipitate the fine uniform xcex1 phase easily for its high strength, with the matrix kept in the work-hardened state resulting from hot working, whereas the easily precipitated xcex1 phase produces an adverse effect in the course of working.
The present invention was made in view of the foregoing. An object of the present invention is to provide a titanium alloy, particularly near-xcex2 alloy and xcex2 alloy, having high strength, high ductility, and high toughness, suitable for use as aircraft engine parts and golf club face, while permitting coil-rolling and coiling at a high temperature for high productivity. Another object of the present invention is to provide a process for producing efficiently and certainly such a titanium alloy having remarkable functional properties.
The first aspect of the present invention resides in a process for producing a titanium alloy which comprises heating a xcex2 titanium alloy or near-xcex2 titanium alloy containing not more than 1.0% (excluding 0%) of Si and subjecting said alloy to plastic deformation while keeping silicides solved in it at a temperature above the xcex2-transus, so that silicides precipitate in the form of fine particles, with recrystallization suppressed. This process may be used to produce a titanium alloy which has good workability and exhibits high strength after aging treatment. (xe2x80x9c%xe2x80x9d means xe2x80x9cmass %xe2x80x9d throughout this specification.)
The second aspect of the present invention resides in a process for producing a high-strength titanium alloy which comprises performing hot working on a xcex2 titanium alloy or near-xcex2 titanium alloy containing not more than 1.0% (excluding 0%) of Si such that the hot working finishes at a temperature lower than the solvus of silicides and subsequently performing aging treatment (including annealing) or both solution treatment and aging treatment (including annealing) in the two-phase region at a temperature below the xcex2-transus, without heating above said solvus, thereby causing the acicular xcex1 phase to precipitate almost all over the xcex2 phase matrix.
In this process, said hot working may be followed by heating to a temperature above the xcex2-transus and below the solvus of silicides before the aging treatment (including annealing) or both the solution treatment and aging treatment (including annealing). Heating in this way causes the xcex1 phase to precipitate in a fine uniform acicular form in the xcex2 phase which has not yet recrystallized. Thus, the resulting titanium alloy has high strength due to precipitation hardening.
The above-mentioned process may be applied to the production of a titanium alloy from a titanium alloy ingot. In this case, hot working is carried out such that it finishes at a temperature below the solvus of silicides and hot working is followed by heat treatment at a temperature above the precipitation temperature of silicides. The heat treatment in this manner causes fine precipitates of silicides to form a solid solution once and the xcex2 phase recrystallizes to become fine crystal grains, so that the subsequent precipitation of the fine suicides and the acicular xcex1 phase add to the strength and toughness of the titanium alloy after aging treatment.
The titanium alloy pertaining to the present invention is a xcex2 alloy or near-xcex2 alloy containing not more than 1.0% (excluding 0%) of Si which is characterized in that said Si is present in the form of uniformly dispersed precipitant of silicides having a particle size smaller than 1 xcexcm (excluding 0 xcexcm). According to a preferred embodiment, the alloy contains the acicular a phase which precipitates substantially throughout the xcex2 phase matrix, so that the alloy exhibits remarkable strength, ductility, and toughness.
The titanium alloy specified above may contain not more than 10% of Sn, so that its age hardening is delayed. The resulting alloy is exempt from age hardening due to remaining heat after coil rolling and troubles with recoiling. This enables continuous rolling (coil rolling) and greatly improves the post-rolling steps.
The present invention produces its full effect when the alloy contains xcex2-stabilizing elements as much as specified by the formula below.
0.60xe2x89xa6% Mo/10+% V/15+% Fe/4+% Cr/8+% Mn/6+% Co/6+% Ni/8+% W/25+% Nb/36+% Ta/50xe2x89xa62.0 
A preferred titanium alloy of the present invention is a near-xcex2 titanium alloy containing Mo 13-17%, Zr 3-7%, and Al 1.5-4.5% (typically a Ti-15Mo-5Zr-3Al-3Sn alloy).
Another preferred titanium alloy of the present invention is a near-xcex2 titanium alloy containing Al 3-7%, Mo 2-6%, Cr 2-6%, and Zr 1-6% (typically a Ti-5Al-2Sn-2Zr-4Mo-4Cr alloy).